1. Field of the Invention
The present invention generally relates to a method for inhibiting a variety of diseases in plants. More specifically, the present invention is directed to methods for inhibiting diseases in plants by applying an aqueous solution of urea and at least one water-soluble salt of a divalent cation and a monovalent anion to the seeds, stems, leaves, seed heads or sub-surface root zone soil of said plants.
2. Description of the Background
A variety of metal-containing solutions have been used over the years to control bacterial and fungal disease in plants. Examples of metallic compounds used in these solutions include tribasic copper sulfate, copper oxychloride, copper hydroxide, Bordeaux solutions and the bis dithiocarbomates of zinc, iron and manganese. These metallic compounds are substantially insoluble in water and protect the crops from the outside surface of the plant leaves and stems where they inhibit growth of various bacteria and fungi. If these compounds were water soluble, they would be toxic to plant tissue in the concentrations in which they are applied.
Water soluble compounds, including calcium nitrate, calcium chloride and calcium sulfate, have been reported to control soft rot on potatoes and club root on brassica crops when applied to the surface of surrounding soil. The inventor is unaware of any compound of magnesium which has been reported to provide any fungal or bacterial control.
In summary, it is known that soluble calcium compounds may be used to control soil borne diseases when soil applied while several non-soluble metallic compounds have been reported to control foliar diseases when applied to the surfaces of leaves and stems of plants.
While urea has not been used to control plant disease, urea, being approximately 46 percent-by-weight nitrogen, has long been preferred as a nitrogen source for fertilizing soils to stimulate plant growth. However, high ammonia losses when used in the presence of moisture effectively restricted use of urea for many years. It is believed that these losses are caused by the hydrolysis of urea, in the presence of moisture and the enzyme urease, according to the following reaction: ##STR1## However, when urea is mixed in a solution with a water-soluble salt comprising a divalent alkaline earth metal or a divalent transition metal and a monovalent anion, the urease activity illustrated in the above equation is inhibited. In order to inhibit this urease activity, it has been found that the soluble salt must comprise a divalent cation, together with a monovalent anion. Exemplary inhibiting salts are the chlorides and nitrates of calcium, magnesium, manganese and zinc. The inventor believes that these salts inhibit urease activity according to the following reaction: ##STR2## Aqueous solutions of urea and the described soluble salts have been proposed for use as fertilizers. See the proposal in British Patent No. 1,561,136 which, while directed to the use of these solutions as liquid nutrient supplements for animal feeds, suggests their use for plant fertilization. Also, see U.S. Pat. No. 4,500,335 which proposes the application of urea in the presence of salts of calcium or magnesium as a method for reducing ammonia volatilization from surface applied urea-containing fertilizers.
However, no known references have suggested that these solutions be directly applied to plants for the purpose of inhibiting various fungal and bacterial diseases. Accordingly, while similar solutions have been generally known, the agricultural industry has continued to employ a variety of toxic chemicals, including those mentioned in the first paragraph above, to control fungal and bacterial diseases in plants. There has been a long felt but unfulfilled need in the industry for an effective, but non-toxic, method of inhibiting growth of fungal and bacterial diseases in a variety of crop plants. The present invention solves that need.